High liquidus viscosity glasses for flat panel displays

ABSTRACT

This invention is related to glasses for use as substrates in flat panel display devices; more specifically, for use as substrates in LCDs which utilize polycrystalline silicon thin film transistors. The compositions for the inventive glasses are essentially free from alkali metal oxides and consist essentially, in mole percent, of -SiO2 64-70 Y2O3 0-5 -Al2O3 9.5-14  MgO 0-5 -B2O3  5-10 CaO  3-13 -TiO2 0-5 SrO   0-5.5 -Ta2O5 0-5 BaO 2-7 -Nb2O5 0-5 MgO + CaO + SrO + BaO  10-20. -

This application is a continuation-in-part application of applicationSer. No. 08/008,560, filed January 22, 1993, and now abandoned.

RELATED APPLICATION

U.S. application Ser. No. 08/8561, filed concurrently with applicationSer. No. 08/008,560 under the title BARIUM ALUMINOSILICATE GLASSES andnow abandoned, is directed to glasses particularly designed for use assubstrates in liquid crystal display devices employing polysilicon thinfilm transistors as switches. Those glasses exhibit strain points of atleast 660° C., liquidus temperatures no higher than 1175° C.,viscosities at the liquidus temperature of at least 600,000 poises(60,000 Pa.s), and weight losses of less than 1 mg/cm² after immersionfor 24 hours in an aqueous 5% by weight HCl solution at 95° C., theglasses being essentially free from alkali metal oxides and consistingessentially, expressed in terms of mole percent on the oxide basis, of

    __________________________________________________________________________    SiO.sub.2                                                                         65-78                                                                              Mgo        0-5                                                                              ZrO.sub.2    0-2.5                                     Al.sub.2 O.sub.3                                                                   7-11                                                                              CaO        0-10                                                                             TiO.sub.2    0-3                                       BaO 12-19                                                                              SrO        0-10                                                                             Ta.sub.2 O.sub.5                                                                           0-3                                       B.sub.2 O.sub.3                                                                   0-5  MgO + CaO + SrO                                                                          0-15                                                                             ZrO.sub.2 + TiO.sub.2 + Ta.sub.2 O.sub.5                                                   0-5                                       __________________________________________________________________________

1. Field of the Invention

This invention is directed to the production of glass compositionsexhibiting high viscosities at their liquidus temperatures. Moreparticularly, this invention is directed to such glasses demonstratingchemical and physical characteristics rendering them especially suitablefor use as substrates in flat panel display devices.

2. Background of the Invention

Liquid crystal displays (LCDs) are passive displays which depend uponexternal sources of light for illumination. They are manufactured assegmented displays or in one of two basic configurations. The substrateneeds (other than being transparent and capable of withstanding thechemical conditions to which it is exposed during display processing) ofthe two matrix types vary. The first type is intrinsic matrix addressed,relying upon the threshold properties of the liquid crystal material.The second is extrinsic matrix or active matrix (AM) addressed, in whichan array of diodes, metal-insulator-metal (MIM) devices, or thin filmtransistors (TFTs) supplies an electronic switch to each pixel. In bothcases, two sheets of glass form the structure of the display. Theseparation between the two sheets is the critical gap dimension, of theorder of 5-10 μm.

Intrinsically addressed LCDs are fabricated using thin film depositiontechniques at temperatures ≦350° C., followed by photolithographicpatterning. As a result, the substrate requirements therefor are oftenthe same as those for segmented displays. Soda-lime-silica glass with abarrier layer has proven to be adequate for most needs. A highperformance version of intrinsically addressed LCDs, the super twistednematic (STN) type, has an added requirement of extremely preciseflatness for the purpose of holding the gap dimensions uniform. Becauseof that requirement, soda-lime-silica glass used for those displays mustbe polished or, alternatively, a precision formed, bariumaluminoborosilicate glass marketed by Corning Incorporated, Corning, NewYork, as Code 7059 may be used without polishing.

Extrinsically addressed LCDs can be further subdivided into twocategories; viz., one based on MIM or amorphous silicon (a-Si) devices,and the other based on polycrystalline silicon (poly-Si) devices. Thesubstrate requirements of the MIM or a-Si type are similar to the STNapplication. Corning Code 7059 sheet glass is the preferred substratebecause of its very low sodium content, i.e., less than 0.1% Na₂ O byweight, its dimensional precision, and its commercial availability.Devices formed from poly-Si, however, are processed at highertemperatures than those that are employed with a-Si TFTs. Substratescapable of use temperatures (taken to be 25° C. below the strain pointof the glass) of 600°-800° C. are demanded. The actual temperaturerequired is mandated by the particular process utilized in fabricatingthe TFTs. Those TFTs with deposited gate dielectrics require 600°-650°C., while those with thermal oxides call for about 800° C. Both a-Si andpoly-Si processes demand precise alignment of successivephotolithographic patterns, thereby necessitating that the thermalshrinkage of the substrate be kept low. Those temperatures have mandatedthe use of glasses exhibiting higher strain points than soda-lime-silicaglass and Corning Code 7059 glass in order to avoid thermal deformationof the sheet during processing. As can be appreciated, the lower thestrain point, the greater this dimensional change. Thus, there has beenconsiderable research to develop glasses demonstrating high strainpoints so that thermal deformation is minimized during device processingat temperatures greater than 600° C., and preferably, higher than 650°C.

U.S. Pat. No. 4,824,808 (Dumbaugh, Jr.) lists four properties which havebeen deemed mandatory for a glass to exhibit in order to fully satisfythe needs of a substrate for LCDs:

First, the glass must be essentially free of intentionally added alkalimetal oxide to avoid the possibility that alkali metal from thesubstrate can migrate into the transistor matrix;

Second, the glass substrate must be sufficiently chemically durable towithstand the reagents used in the TFT matrix deposition process;

Third, the expansion mismatch between the glass and the silicon presentin the TFT array must be maintained at a relatively low level even asprocessing temperatures for the substrates increase; and

Fourth, the glass must be capable of being produced in high quality thinsheet form at low cost; that is, it must not require extensive grindingand polishing to secure the necessary surface finish.

That last requirement is most difficult to achieve inasmuch as itdemands a sheet glass production process capable of producingessentially finished glass sheet, such as the overflow downdraw sheetmanufacturing process described in U.S. Pat. No. 3,338,696 (Dockerty)and U.S. Pat. No. 3,682,609 (Dockerty). That process requires a glassexhibiting a very high viscosity at the liquidus temperature plus longterm stability, e.g., periods of 30 days, against devitrification atmelting and forming temperatures.

Corning Code 7059 glass, supra, is currently employed in the fabricationof LCDs. That glass, consisting essentially, in weight percent, of about50% SiO₂, 15% B₂ O₃, 10% Al₂ O₃, and 24% BaO, is nominally free ofalkali metal oxides, and exhibits a linear coefficient of thermalexpansion (25°-300° C.) of about 46×10⁻⁷ /° C. and a viscosity at theliquidus temperature in excess of 600,000 poises (6×10⁴ Pa.s). The highliquidus viscosity of the glass enables it to be drawn into sheet viathe overflow downdraw sheet processing technique, but its relatively lowstrain point (˜593° C.) is adequate only for processing a-Si devices andnot for poly-Si devices.

The glasses of U.S. Pat. No. 4,824,808, supra, were designed to meet therequirements for use in fabricating poly-Si devices, including thecapability of being formed into sheet by the overflow downdraw sheetprocessing technique, and linear coefficients of thermal expansion aslow as about 36.5×10⁻⁷ /° C. (25°-300° C.), such as to closely matchthat of silicon, thereby enabling a silicon chip to be sealed directlythereon, but their strain points were less than 650° C.

The glasses of U.S. Pat. No. 4,409,337 (Dumbaugh, Jr.) were alsoconsidered for LCD substrates, but their long term stability againstdevitrification was feared to be insufficient for their use in theoverflow downdraw sheet processing technique.

The glasses of U.S. Pat. No. 5,116,787 (Dumbaugh, Jr.) are essentiallyfree from alkali metal oxides and MgO and demonstrate strain points of655° C. and higher, with viscosities at the liquidus greater than1.5×10⁵ poises (1.5×10⁴ Pa.s). Although designed for use in the overflowdowndraw sheet processing technique, their long term stability againstdevitrification was found to be marginal when employed in the process,some crystallization being formed in the glass during manufacture. Also,the linear coefficients of thermal expansion (25°-300° C.) generallyranged in the upper 40s to low 50s×10⁻⁷ /° C., undesirably high formatching that of silicon (˜35×10⁻⁷ /° C.).

U.S. Pat. No. 5,116,788 (Dumbaugh, Jr.) discloses other glassesexhibiting high strain points, i.e., greater than 675° C., but havingsuch relatively low viscosities at the liquidus temperature, viz.,20,000-200,000 poises (2,000-20,000 Pa.s), as to be subject todevitrification when formed utilizing the overflow downdraw sheetprocessing technique.

Finally, U.S. Pat. No. 5,116,789 (Dumbaugh, Jr. et al.) describes otherglass compositions demonstrating strain points higher than 675° C., buttheir linear coefficients of thermal expansion (25°-300° C.) range from45-62×10⁻⁷ /° C., too high to form a high temperature match withsilicon.

A recent advance in liquid crystal technology termed "chip-on-glass"(COG) has further emphasized the need for the substrate glass to closelymatch silicon in thermal expansion. Thus, the initial LCD devices didnot have their driver chips mounted on the substrate glass. Instead, thesilicon chips were mounted remotely and were connected to the LCDsubstrate circuitry with compliant or flexible wiring. As LCD devicetechnology improved and as the devices became larger, these flexiblemountings became unacceptable, both because of cost and of uncertainreliability. This situation led to Tape Automatic Bonding (TAB) of thesilicon chips. In that process the silicon chips and electricalconnections to the chips were mounted on a carrier tape, thatsubassembly was mounted directly on the LCD substrate, and thereafterthe connection to the LCD circuitry was completed. TAB decreased costwhile improving reliability and increasing the permitted density of theconductors to a pitch of approximately 200 μm--all significant factors.COG, however, provides further improvement over TAB with respect tothose three factors. Hence, as the size and quality requirements of LCDdevices increase, COG is demanded for those devices dependent upon theuse of integrated circuit silicon chips. For that reason, the substrateglass must demonstrate a linear coefficient of thermal expansion closelymatching that of silicon; i.e., the glass must exhibit a linearcoefficient of thermal expansion (0°-300° C.) between 32-46×10⁻⁷ /° C.,most preferably 32-40×10⁻⁷ /° C.

SUMMARY OF THE INVENTION

The present invention is founded in the discovery of glass exhibitinglinear coefficients of thermal expansion over the temperature range of0°-300° C. between 32-46×10⁻⁷ /° C., strain points higher than 650° C.,liquidus temperatures no higher than 1200° C., liquidus viscositiesgreater than about 200,000 poises (20,000 Pa.s), a weight loss of lessthan 2 mg/cm² after immersion for 24 hours in an aqueous 5% by weightHCl solution at 95° C., long term stability against devitrification atmelting and forming temperatures, and melting viscosities of about 200poises (20 Pa.s) at less than 1675° C., the glasses being essentiallyfree from alkali metal oxides and consisting essentially, expressed interms of mole percent on the oxide basis, of

    ______________________________________                                        SiO.sub.2                                                                           64-70     Y.sub.2 O.sub.3    0-3                                        Al.sub.2 O.sub.3                                                                    9.5-14    MgO                0-5                                        B.sub.2 O.sub.3                                                                      5-10     CaO                 3-13                                      TiO.sub.2                                                                           0-5       SrO                  0-5.5                                    Ta.sub.2 O.sub.5                                                                    0-5       BaO                2-7                                        Nb.sub.2 O.sub.5                                                                    0-5       MgO + CaO + SrO + BaO                                                                            10-20                                      ______________________________________                                    

Because of their deleterious effect upon the electrical properties ofthe glasses, the alkali metal oxides will most preferably be totallyabsent therefrom.

Adherence to those composition intervals has been found to be vital insecuring glasses demonstrating properties desired for LCD substrateglasses to be employed in poly-Si devices. To illustrate:

If the SiO₂ content is below 64%, the strain point will be too low andthe resistance of the glass to attack by acid will be poor. On the otherhand, when the concentration of SiO₂ exceeds 70%, the glass becomesdifficult to melt at customary glass melting temperatures.

The level of Al₂ O₃ is critical to obtain a minimum liquidustemperature; that is, if the concentration of Al₂ O₃ falls below 9.5% orexceeds 14%, the liquidus temperature can exceed 1200° C.

B₂ O₃ tends to improve the viscosity character of the glass therebymaking it easier to process. Where the level of B₂ O₃ is less than 5%,the liquidus temperature becomes too high. At contents above 10% B₂ O₃,the resistance to attack by acid is sacrificed and the strain pointbecomes too low.

The alkaline earth metal oxides modify the melting and physicalproperties of the glass. If the total thereof exceeds 20%, however, thelinear coefficient of thermal expansion becomes too high. SrO and BaOraise the thermal expansion more than either MgO or CaO and, hence, willindividually not exceed 7%. Because CaO also raises the thermalexpansion, but at a slower rate than SrO or BaO, the total thereof oughtnot to exceed 13%. MgO appears to exert a beneficial effect upon theliquidus temperature at low levels, but, at concentrations greater than5%, the liquidus temperature appears to rise.

The inclusion of TiO₂ is useful in lowering the coefficient of thermalexpansion of the glass. The presence of Ta₂ O₅ not only lowers thecoefficient of thermal expansion of the glass but also raises the strainpoint of the glass and significantly improves the chemical durabilitythereof. Nb₂ O₅ and Y₂ O₃ appear to reduce the coefficient of thermalexpansion of the glass and lower the liquidus temperature thereof.

In the preferred glasses the level of Al₂ O₃ will exceed that of B₂ O₃and in the most preferred glasses the composition will consistessentially, expressed in terms of mole percent, of about

    ______________________________________                                        SiO.sub.2                                                                           65-69      MgO               1-5                                        Al.sub.2 O.sub.3                                                                    10-12      CaO               3-9                                        B.sub.2 O.sub.3                                                                      7-10      SrO               1-3                                        TiO.sub.2                                                                           0-3        BaO               2-5                                        Ta.sub.2 O.sub.5                                                                    0-3        MgO + CaO + SrO + BaO                                                                           11-16                                      ______________________________________                                    

wherein the ratio Al₂ O₃ :B₂ O₃ >1. The glasses will exhibit a linearcoefficient of thermal expansion (0°-300° C.) of 32-40×10⁻⁷ /° C., aliquidus temperature no higher than 1125° C., and a viscosity at theliquidus temperature greater than 600,000 poises (60,000 Pa.s).

DESCRIPTION OF PREFERRED EMBODIMENTS

Table I records a number of glass compositions, expressed in terms ofparts by weight on the oxide basis, illustrating the compositionalparameters of the present invention. Inasmuch as the sum of theindividual constituents totals or very closely approximates 100, for allpractical purposes the reported values may be deemed to represent weightpercent. The actual batch ingredients may comprise any materials, eitheroxides or other compounds, which, when melted together with the otherbatch components, will be converted into the desired oxide in the properproportions. For example, SrCO₃ and CaCO₃ can provide the source of SrOand CaO, respectively.

The batch ingredients were compounded, tumble mixed together thoroughlyto aid in producing a homogeneous melt, and charged into platinumcrucibles. After placing lids thereon, the crucibles were moved intofurnaces operating at temperatures of 1600°-1650° C. To assure theproduction of inclusion- and cord-free glasses a two step meltingprocedure was used. In the first step the batch was melted for about 16hours, stirred and then poured as a fine stream into a water bath, aprocess termed "drigaging" in the glass art. In the second step thefinely-divided glass particles from the drigaging were remelted at1600°-1650° C. for about four hours, the melts stirred in bothdirections (clockwise and counterclockwise), the melts thereafter pouredonto steel plates to make glass slabs having dimensions of about18"×6"×0.5" (˜45.7×15.2×1.3 cm), and those slabs transferred immediatelyto an annealer operating at about 730° C.

Whereas the above description reflects a laboratory melting procedure,it must be appreciated that the inventive glasses are capable of beingmelted and formed employing large scale, commercial glass melting andforming equipment. Thus, the glasses are specifically designed to bedrawn into thin sheet utilizing the overflow downdraw sheet processingtechnique. For the experimental glasses described herein, arsenic and/orantimony in amounts of about 0-1% and 0-0.5%, respectively, were addedto each batch to perform their customary function as a fining agent. Thesmall residual remaining in the glass has no substantial effect upon theproperties of the glass.

Table I also lists measurements of several chemical and physicalproperties determined on the glasses in accordance with techniquesconventional in the glass art. Thus, the linear coefficient of thermalexpansion (Exp.) over the temperature range 0°-300° C. expressed interms of ×10⁻⁷ /° C., and the softening point (S.P.), annealing point(A.P.), and strain point (St.P.) expressed in terms of ° C., weredetermined by fiber elongation. The durability (Dur.) in HCl wasdetermined by measuring the weight loss (mg/cm²) after immersion in abath of aqueous 5% by weight HCl at 95° C. for 24 hours.

The liquidus temperatures of the glasses were measured via two differentmethods. The standard liquidus method (Liq.) involves placing crushedglass particles in a platinum boat, placing the boat in a furnace havinga region of gradient temperatures, heating the boat in an appropriatetemperature region for 24 hours, and determining by means of microscopicexamination the highest temperature at which crystals appear in theinterior of the glass. A second method termed the "meltback liquidus"(M.Liq.) contemplates placing a glass sample which has beenprecrystallized by holding at a temperature of 1000° C. for 24 hours ina platinum boat, heating the boat in an appropriate temperature regionin a gradient furnace for 24 hours, and then determining by microscopicexamination the lowest temperature at which crystals are not observed inthe interior of the glass. Generally, the liquidus temperatures measuredby these two techniques do not differ by more than 50° C., with the"meltback liquidus" typically being higher than the standard liquidustemperature.

Table IA records the same glass compositions expressed in terms of molepercent on the oxide basis, the content of the fining agents beingomitted as their presence in the final glass is small and has nosubstantive effect upon the overall properties of the glass.

    __________________________________________________________________________         1    2    3    4    5    6    7                                          __________________________________________________________________________    SiO.sub.2                                                                          57.0 57.3 56.8 59.3 55.0 59.2 55.8                                       Al.sub.2 O.sub.3                                                                   15.1 14.7 15.3 15.7 14.5 15.6 17.3                                       B.sub.2 O.sub.3                                                                    8.57 8.61 8.55 4.89 8.28 8.90 8.59                                       MgO  0.70 0.70 0.70 0.73 0.68 0.73 0.702                                      CaO  7.64 7.67 7.61 7.95 10.8 4.13 6.51                                       SrO  1.80 1.81 1.80 1.87 1.74 1.87 1.81                                       BaO  9.11 9.15 9.08 9.48 8.79 9.45 9.13                                       As.sub.2 O.sub.3                                                                   0.3  0.3  0.3  0.3  0.3  0.3  0.3                                        Sb.sub.2 O.sub.3                                                                   0.5  0.5  0.5  0.5  0.5  0.5  0.5                                        Exp. 43.0 42.9 43.1 43.7 47.4 38.1 41.0                                       S.P. 924  922  926  961  894  972  950                                        A.P. 705  703  704  733  725  725  722                                        St.P.                                                                              656  653  656  680  669  668  671                                        Dur. 1.12 --   --   --   1.94 1.88 --                                         M.Liq.                                                                             1075 1080 1085 1150 1080 1070 1115                                       __________________________________________________________________________         8    9    10   11   12   13   14   15                                    __________________________________________________________________________    SiO.sub.2                                                                          57.1 57.1 59.1 58.5 57.2 57.8 56.4 59.4                                  Al.sub.2 O.sub.3                                                                   14.7 14.8 15.6 15.6 16.5 15.4 16.0 17.4                                  B.sub.2 O.sub.3                                                                    6.22 7.0  8.31 8.3  9.53 8.2  7.48 10.0                                  MgO  0.285                                                                              0.432                                                                              1.42 1.42 0.73 1.4  1.38 1.45                                  CaO  9.72 9.01 4.13 4.13 3.18 4.08 4.01 6.05                                  SrO  0.733                                                                              1.11 1.87 1.87 1.87 1.84 1.81 *0.09                                 BaO  11.1 10.4 9.46 9.44 9.48 9.33 9.18 5.51                                  TiO.sub.2                                                                          --   --   --   0.69 1.38 --   --   --                                    Ta.sub.2 O.sub.5                                                                   --   --   --   --   --   1.87 3.68 --                                    Exp. 45.9 44.8 37.7 37.4 35.0 38   36.5 34.6                                  S.P. 925  924  969  964  971  969  973  966                                   A.P. 713  711  723  721  725  728  739  730                                   St.P.                                                                              667  660  667  664  667  670  681  676                                   Dur. 0.3  0.4  0.86 0.7  1.28 0.6  0.28 1.6                                   M.Liq.                                                                             1095 1110 --   --   --   --   --   --                                    Liq. --   --   1065 1060 1090 1055 1055 1085                                  __________________________________________________________________________     *Impurity in BaCO.sub.3 batch material. None purposely added.            

                                      TABLE IA                                    __________________________________________________________________________         1    2    3    4    5    6    7                                          __________________________________________________________________________    SiO.sub.2                                                                          65.4 65.6 65.3 68.0 62.7 68.5 64.8                                       Al.sub.2 O.sub.3                                                                   10.2 9.9  10.4 10.6 9.8  10.7 11.8                                       B.sub.2 O.sub.3                                                                    8.55 8.5  8.5  4.8  8.1  8.9  8.61                                       MgO  1.2  1.2  1.2  1.2  1.1  1.2  1.22                                       CaO  9.4  9.4  9.4  9.8  13.2 5.1  8.1                                        SrO  1.2  1.2  1.2  1.2  1.1  1.2  1.22                                       BaO  4.1  4.1  4.1  4.2  3.9  4.3  4.15                                       __________________________________________________________________________         8    9    10   11   12   13   14   15                                    __________________________________________________________________________    SiO.sub.2                                                                          65.8 65.7 68.1 67.5 66.5 67.7 67.3 66.7                                  Al.sub.2 O.sub.3                                                                   10.0 10.0 10.6 10.6 11.3 10.6 11.2 11.5                                  B.sub.2 O.sub.3                                                                    6.19 6.95 8.26 8.26 8.96 8.29 7.71 9.70                                  MgO  0.49 0.74 1.25 1.25 1.26 1.25 1.26 2.42                                  CaO  12.0 11.1 5.1  5.1  5.16 5.11 5.13 7.27                                  SrO  0.49 0.74 1.25 1.25 1.26 1.25 1.26 --                                    BaO  5.02 4.7  4.27 4.27 4.32 4.28 4.29 2.42                                  TiO.sub.2                                                                          --   --   --   0.59 1.2  --   --   --                                    Ta.sub.2 O.sub.5                                                                   --   --   --   --   --   0.3  0.6  --                                    __________________________________________________________________________

Example 5 illustrates how a glass with high CaO and low SiO₂ can resultin high thermal expansion coefficients.

Table II records a second group of glass compositions which werecharacterized more in depth, again expressed in terms of parts by weighton the oxide basis. Again, because the sum of the individualconstituents totals or very closely approximates 100, for all practicalpurposes the values recited may be considered to reflect weight percent.And, in like manner to the compositions recorded in Table I, the actualbatch ingredients may comprise any materials, either oxides or othercompounds, which, when melted together, will be converted into thedesired oxide in the proper proportions.

The batch ingredients were compounded, mixed together, melted, andshaped into glass slabs in accordance with the laboratory procedureoutlined above for the glasses of Table I.

Table II also lists the measurements of chemical and physical propertiesconducted on the several glasses along with two additionaldeterminations not performed on the glasses of Table I. First, themelting temperature [Melt, in ° C.] (defined as the temperature at whichthe glass melt demonstrates a viscosity of 200 poises [20 Pa.s]) hasbeen added, being calculated employing the Fulcher equation as fit tothe high temperature viscosity data. Second, the liquidus viscosity(Visc), also calculated using the Fulcher equation coefficients, wasadded, expressed in terms of ×100,000 poises (10,000 Pa.s). The liquidustemperature reported was determined in accordance with the standardliquidus method (Liq.). Finally, in like manner to the glasscompositions of Table I, fining of the glass was accomplished byincluding arsenic and/or antimony in the glass batch.

Table IIA reports the same glass compositions expressed in terms of molepercent on the oxide basis, the concentration of fining agent beingomitted because the small residual therefrom has no substantive effectupon the overall properties of the glass. Example No. 16 is a laboratoryremelt of Example No. 6.

                                      TABLE II                                    __________________________________________________________________________         16   17   18   19   20   21   22   23                                    __________________________________________________________________________    SiO.sub.2                                                                          59.2 57.7 59.9 58.8 59.6 58.3 58.2 57.8                                  Al.sub.2 O.sub.3                                                                   15.6 15.2 15.8 15.5 15.8 16.5 16.5 17.0                                  B.sub.2 O.sub.3                                                                    8.90 8.67 9.01 8.84 8.96 8.94 8.32 8.94                                  MgO  0.73 0.71 0.74 --   --   0.73 1.42 0.73                                  CaO  4.13 6.57 2.99 5.45 4.18 4.15 4.14 4.15                                  SrO  1.87 1.82 1.89 1.86 1.88 1.87 1.87 1.87                                  BaO  9.45 9.21 9.57 9.39 9.52 9.49 9.47 9.49                                  Exp. 37.5 42.0 36.2 39.5 37.2 38.1 37.1 37.2                                  S.P. 973  937  992  972  989  980  972  979                                   A.P. 721  707  730  722  732  731  730  735                                   St.P.                                                                              662  656  669  666  672  673  673  677                                   Dur. 0.62 1.04 0.52 0.63 0.52 0.72 1.1  1.12                                  Liq. 1065 1050 1085 1105 1070 1035 1020 1030                                  Melt 1645 1560 1670 1644 1670 1632 1619 --                                    Visc 21   8.8  22   6.7  32   58   79   --                                    __________________________________________________________________________         24   25   26   27   28   29   30   31                                    __________________________________________________________________________    SiO.sub.2                                                                          61.3 58.4 58.1 57.3 57.5 55.4 54.5 48.0                                  Al.sub.2 O.sub.3                                                                   14.4 17.1 16.5 16.5 12.5 16.0 16.3 12.8                                  B.sub.2 O.sub.3                                                                    9.8  9.85 7.11 8.35 7.5  7.07 7.22 6.73                                  MgO  1.42 1.43 2.8  2.12 --   4.43 3.13 --                                    CaO  3.95 3.97 4.13 4.15 2.5  2.37 8.23 10.8                                  SrO  3.65 3.67 1.87 1.88 --   7.01 --   8.58                                  BaO  5.4  5.42 9.45 9.5  20.0 7.78 10.6 10.7                                  Exp. 37.2 35.8 39.1 38.6 43.9 41.3 44.5 59.4                                  S.P. 970  969  953  957  917  --   908  --                                    A.P. 719  730  722  726  702  719  703  670                                   St.P.                                                                              664  674  670  676  653  674  657  627                                   Dur. --   1.8  0.62 0.88 0.39 0.9  2.69 2.9                                   Liq. --   1050 1075 1055 1080 1242 1086 1042                                  Melt --   1610 1585 1595 1758 --   1475 --                                    Visc --   29   8.0  17   14.8 --   1.0  --                                    __________________________________________________________________________         32   33   34   35   36   37   38                                         __________________________________________________________________________    SiO.sub.2                                                                          57.5 57.2 58.2 57.1 55.69                                                                              57.3 57.3                                       Al.sub.2 O.sub.3                                                                   18.2 18.6 18.4 17.6 19.6 19.0 17.9                                       B.sub.2 O.sub.3                                                                    7.41 7.08 8.14 7.65 7.75 6.79 7.38                                       MgO  1.42 1.76 1.44 1.41 1.43 1.42 1.41                                       CaO  4.13 4.11 4.18 4.1  4.14 4.11 4.11                                       SrO  1.87 1.86 --   --   1.88 1.86 1.86                                       BaO  9.45 9.41 9.57 12.1 9.49 9.43 10.1                                       Exp. 36.9 37.2 34.9 38.1 37.6 36.6 37.8                                       S.P. 987  983  988  981  982  990  983                                        A.P. 740  740  738  734  741  749  743                                        St.P.                                                                              682  682  678  674  684  693  690                                        Dur. 0.7  0.78 0.83 0.77 1.84 0.66 0.64                                       Liq. 1080 1100 1090 1035 1145 1155 1090                                       Melt 1600 1582 1607 1597 1581 1589 1593                                       Visc 14   8    12   >40  2.2  2.4  11                                         __________________________________________________________________________         39   40   41   42   43   44   45   46                                    __________________________________________________________________________    SiO.sub.2                                                                          56.3 56.8 57.2 55.8 56.4 55.3 55.5 55.7                                  Al.sub.2 O.sub.3                                                                   18.3 18.4 18.6 17.7 17.9 16.6 16.6 16.7                                  B.sub.2 O.sub.3                                                                    6.38 6.43 6.48 6.32 6.98 7.55 7.58 7.61                                  MgO  1.39 1.4  1.41 1.38 1.39 2.07 2.08 2.08                                  CaO  4.04 4.07 5.06 4.0  4.05 4.04 4.06 4.07                                  SrO  1.83 3.6  1.86 1.81 --   1.83 1.84 1.84                                  BaO  11.83                                                                              9.33 9.4  13.0 13.4 9.26 9.3  9.33                                  Ta.sub.2 O.sub.5                                                                   --   --   --   --   --   2.79 1.87 0.94                                  Nb.sub.2 O.sub.5                                                                   --   --   --   --   --   0.56 1.12 1.69                                  Exp. 40.1 39.1 38.4 41.2 38.6 38.4 38.4 38.1                                  S.P. 985  986  982  978  985  957  956  951                                   A.P. 746  737  742  740  744  729  728  724                                   St.P.                                                                              690  681  688  687  690  676  674  671                                   Dur. 0.63 0.57 0.57 0.53 0.59 0.56 0.65 0.7                                   Liq. 1130 1060 1120 1140 1080 1070 1065 1070                                  Melt 1587 1597 1583 1586 1601 --   --   --                                    Visc 4    30   4.5  2.6  14   --   --   --                                    __________________________________________________________________________         47   48   49   50   51                                                   __________________________________________________________________________    SiO.sub.2                                                                          55.9 55.4 55.6 55.9 56.0                                                 Al.sub.2 O.sub.3                                                                   16.8 16.6 16.6 16.7 16.8                                                 B.sub.2 O.sub.3                                                                    7.64 7.56 7.6  7.63 7.65                                                 MgO  2.09 2.07 2.08 2.09 2.1                                                  CaO  4.09 4.05 4.06 4.08 4.1                                                  SrO  1.85 1.83 1.84 1.85 1.85                                                 BaO  9.37 9.27 9.31 9.35 9.38                                                 Ta.sub.2 O.sub.5                                                                   --   2.79 1.87 0.94 --                                                   Nb.sub.2 O.sub.5                                                                   2.26 --   --   --   1.13                                                 Y.sub.2 O.sub.3                                                                    --   0.48 0.96 1.44 0.96                                                 Exp. 38.3 38.3 38.5 39.2 37.8                                                 S.P. 949  961  959  958  955                                                  A.P. 724  733  731  733  728                                                  St.P.                                                                              671  680  679  681  676                                                  Dur. 0.86 0.64 0.74 0.99 0.97                                                 Liq. 1065 1055 1065 1090 1090                                                 __________________________________________________________________________

                                      TABLE IIA                                   __________________________________________________________________________         16   17   18   19   20   21   22   23                                    __________________________________________________________________________    SiO.sub.2                                                                          68.5 66.4 69.5 68.2 69.4 67.7 67.3 67.3                                  Al.sub.2 O.sub.3                                                                   10.7 10.3 10.8 10.6 10.8 11.3 11.2 11.6                                  B.sub.2 O.sub.3                                                                    8.9  8.6  9.0  8.8  9.0  9.0  8.3  8.99                                  MgO  1.2  1.2  1.3  --   ---  1.3  2.5  1.27                                  CaO  5.1  8.1  3.7  6.8  5.2  5.2  5.1  5.18                                  SrO  1.2  1.2  1.3  1.2  1.3  1.3  1.3  1.27                                  BaO  4.3  4.1  4.4  4.3  4.4  4.3  4.5  4.33                                  __________________________________________________________________________         24   25   26   27   28   29   30   31                                    __________________________________________________________________________    SiO.sub.2                                                                          69.0 66.7 66.5 66.1 70.2 63.6 62.0 57.9                                  Al.sub.2 O.sub.3                                                                   9.52 11.5 11.1 11.2 9.0  10.8 11.0 9.1                                   B.sub.2 O.sub.3                                                                    9.52 9.7  7.03 8.31 7.9  7.0  7.0  7.0                                   MgO  2.38 2.42 4.79 3.65 --   7.58 5.3  --                                    CaO  4.76 4.85 5.07 5.13 3.27 2.91 10.0 14.0                                  Sro  2.38 2.42 1.24 1.26 --   4.66 --   6.0                                   BaO  2.38 2.42 4.24 4.29 9.57 3.49 4.7  6.0                                   __________________________________________________________________________         32   33   34   35   36   37                                              __________________________________________________________________________    SiO.sub.2                                                                          66.9 66.5 67.4 66.9 65.3 66.9                                            Al.sub.2 O.sub.3                                                                   12.5 12.7 12.6 12.2 13.5 13.1                                            B.sub.2 O.sub.3                                                                    7.44 7.09 8.12 7.74 7.83 6.84                                            MgO  2.46 3.04 2.47 2.46 2.49 2.46                                            CaO  5.15 5.11 5.18 5.15 5.21 5.15                                            SrO  1.26 1.25 --   --   1.27 1.26                                            BaO  4.31 4.28 4.33 5.56 4.36 4.31                                            __________________________________________________________________________         38   39   40   41   42   43                                              __________________________________________________________________________    SiO.sub.2                                                                          66.8 66.5 66.5 66.5 66.3 66.7                                            Al.sub.2 O.sub.3                                                                   12.3 12.7 12.7 12.7 12.4 12.4                                            B.sub.2 O.sub.3                                                                    7.42 6.5  6.5  6.5  6.48 7.11                                            MgO  2.45 2.44 2.44 2.44 2.44 2.44                                            CaO  5.14 5.11 5.11 6.31 5.1  5.13                                            SrO  1.26 1.25 2.44 1.25 1.25 --                                              BaO  4.6  5.47 4.28 4.28 6.05 6.14                                            __________________________________________________________________________         44   45   46   47   48   29   50   51                                    __________________________________________________________________________    SiO.sub.2                                                                          65.7 65.7 65.7 65.7 65.7 65.7 65.7 65.7                                  Al.sub.2 O.sub.3                                                                   11.6 11.6 11.6 11.6 11.6 11.6 11.6 11.6                                  B.sub.2 O.sub.3                                                                    7.74 7.74 7.74 7.74 7.74 7.74 7.74 7.74                                  MgO  3.66 3.66 3.66 3.66 3.66 3.66 3.66 3.66                                  CaO  5.15 5.15 5.15 5.15 5.15 5.15 5.15 5.15                                  SrO  1.26 1.26 1.26 1.26 1.26 1.26 1.26 1.26                                  BaO  4.31 4.31 4.31 4.31 4.31 4.31 4.31 4.31                                  Ta.sub.2 O.sub.5                                                                   0.45 0.3  0.15 --   0.45 0.3  0.15 --                                    Nb.sub.2 O.sub.5                                                                   0.15 0.3  0.45 0.6  --   --   --                                         Y.sub.2 O.sub.3                                                                    --   --   --   --   0.15 0.3  0.45 0.3                                   __________________________________________________________________________

Examples 28-31 illustrate glasses having compositions close to, butsomewhat outside of, the inventive ranges. Thus, the SiO₂ and BaOconcentrations are too high in Example 28; the level of SiO₂ is low andthat of MgO too high in Example 29; the content of SiO₂ is low inExample 30; and the amount of SiO₂ is low and that of CaO too high inExample 31.

Examples 16, 21, 22, 25, 26, and 27 constitute preferred compositionswith Example 21 being the most preferred based upon its overallcombination of chemical, physical, and melting properties.

We claim:
 1. A substrate for a flat panel display device wherein saidsubstrate is comprised of a flat, transparent glass exhibiting a strainpoint higher than 650° C., a linear coefficient of thermal expansionover the temperature range 0°-300° C. between 32-46×10⁻⁷ /° C., and aweight loss of less than 2 mg/cm² after immersion for 24 hours in anaqueous 5% by weight HCl solution, said glass being essentially freefrom alkali metal oxides and consisting essentially, expressed in termsof mole percent on the oxide basis, of

    ______________________________________                                        SiO.sub.2                                                                            64-70    Y.sub.2 O.sub.3     0-5                                       Al.sub.2 O.sub.3                                                                     9.5-14   MgO                 0-5                                       B.sub.2 O.sub.3                                                                       5-10    CaO                  3-13                                     TiO.sub.2                                                                            0-5      SrO                   0-5.5                                   T.sub.2 O.sub.5                                                                      0-5      BaO                 2-7                                       Nb.sub.2 O.sub.5                                                                     0-5      MgO + CaO + SrO + BaO                                                                              10-20.                                   ______________________________________                                    


2. A substrate according to claim 1 wherein the liquidus temperature ofsaid glass is no higher than 1200° C.
 3. A substrate according to claim1 wherein the viscosity of said glass at the liquidus temperaturethereof is greater than about 20,000 Pa.s.
 4. A substrate according toclaim 1 wherein said glass exhibits a melting viscosity of about 20 Pa.sat a temperature below 1675° C.
 5. A substrate according to claim 1wherein said glass exhibits a linear coefficient of thermal expansionbetween 32-40×10⁻⁷ /° C.
 6. A substrate according to claim 5 whereinsilicon chips are mounted directed onto said glass utilizingchip-on-glass technology.
 7. A substrate according to claim 5 whereinsaid glass is essentially free from alkali metal oxides and consistsessentially, expressed in terms of mole percent on the oxide basis, of

    ______________________________________                                        SiO.sub.2                                                                            65-69    Y.sub.2 O.sub.3     0-3                                       Al.sub.2 O.sub.3                                                                     10-12    MgO                 1-5                                       B.sub.2 O.sub.3                                                                       7-10    CaO                 3-9                                       TiO.sub.2                                                                            0-3      SrO                 1-3                                       Ta.sub.2 O.sub.5                                                                     0-3      BaO                 2-5                                       Nb.sub.2 O.sub.5                                                                     0-3      MgO + CaO + SrO + BaO                                                                             11-16                                     ______________________________________                                    

wherein the ratio Al₂ O₃ :B₂ O₃ >1.
 8. A substrate according to claim 7wherein the liquidus temperature of said glass is no higher than 1125°C.
 9. A substrate according to claim 7 wherein the viscosity of saidglass at the liquidus temperature thereof is greater than 60,000 Pa.s.10. A substrate according to claim wherein said glass exhibits a meltingviscosity of about 20 Pa.s at a temperature below 1675° C.
 11. Asubstrate according to claim 7 wherein silicon chips are mounteddirectly onto said glass utilizing chip-on-glass technology.
 12. A glassexhibiting a strain point higher than 650° C., a linear coefficient ofthermal expansion over the temperature range 0°-300° C. between32-46×10⁻⁷ /° C., and a weight loss of less than 2 mg/cm² afterimmersion for 24 hours in an aqueous 5% by weight HCl solution, saidglass being essentially free from alkali metal oxides and consistingessentially, expressed in terms of mole percent on the oxide basis, of

    ______________________________________                                        SiO.sub.2                                                                            64-70    Y.sub.2 O.sub.3     0-5                                       Al.sub.2 O.sub.3                                                                     9.5-14   MgO                 0-5                                       B.sub.2 O.sub.3                                                                       5-10    CaO                  3-13                                     TiO.sub.2                                                                            0-5      SrO                   0-5.5                                   Ta.sub.2 O.sub.5                                                                     0-5      BaO                 2-7                                       Nb.sub.2 O.sub.5                                                                     0-5      MgO + CaO + SrO + BaO                                                                              10-20.                                   ______________________________________                                    


13. A glass according to claim 12 wherein the liquidus temperaturethereof is no higher than 1200° C.
 14. A glass according to claim 12wherein the viscosity thereof at the liquidus temperature is greaterthan about 20,000 Pa.s.
 15. A glass according to claim 12 wherein saidglass exhibits a melting viscosity of about 20 Pa.s at a temperaturebelow 1675° C.
 16. A glass according to claim 12 wherein said glassexhibits a linear coefficient of thermal expansion between 32-40×10⁻⁷ /°C.
 17. A glass according to claim 16 wherein said glass consistsessentially of

    ______________________________________                                        SiO.sub.2                                                                            65-69    Y.sub.2 O.sub.3     0-3                                       Al.sub.2 O.sub.3                                                                     10-12    MgO                 1-5                                       B.sub.2 O.sub.3                                                                       7-10    CaO                 3-9                                       TiO.sub.2                                                                            0-3      SrO                 1-3                                       Ta.sub.2 O.sub.5                                                                     0-3      BaO                 2-5                                       Nb.sub.2 O.sub.5                                                                     0-3      MgO + CaO + SrO + BaO                                                                             11-16                                     ______________________________________                                         wherein the ratio Al.sub.2 O.sub.3 :B.sub.2 O.sub.3 >1.


18. A glass according to claim 17 wherein the liquidus temperaturethereof is no higher than 1125° C.
 19. A glass according to claim 17wherein the viscosity thereof at the liquidus temperature is greaterthan 60,000 Pa.s.
 20. In a flat panel display device containing a flat,transparent glass substrate, the improvement wherein said glass exhibitsa strain point higher than 650° C., a linear coefficient of thermalexpansion over the temperature range of 0°-300° C. between 32-46×10⁻⁷ /°C., and a weight loss of less than 2 mg/cm² after immersion for 24 hoursin an aqueous solution of 5% by weight HCl, said glass being essentiallyfree from alkali metal oxides and consisting essentially, expressed interms of mole percent on the oxide basis, of

    ______________________________________                                        SiO.sub.2                                                                            64-70    Y.sub.2 O.sub.3     0-5                                       Al.sub.2 O.sub.3                                                                     9.5-14   MgO                 0-5                                       B.sub.2 O.sub.3                                                                       5-10    CaO                  3-13                                     TiO.sub.2                                                                            0-5      SrO                   0-5.5                                   T.sub.2 O.sub.5                                                                      0-5      BaO                 2-7                                       Nb.sub.2 O.sub.5                                                                     0-5      MgO + CaO + SrO + BaO                                                                              10-20.                                   ______________________________________                                    


21. In a flat panel display device according to claim 20 wherein theliquidus temperature of said glass is no higher than 1200° C.
 22. In aflat panel display device according to claim 20 wherein the viscosity ofsaid glass at the liquidus temperature thereof is greater than about20,000 Pa.s.
 23. In a flat panel display device according to claim 20Wherein said glass exhibits a melting viscosity of about 20 Pa.s at atemperature below 1675° C.
 24. In a flat panel display device accordingto claim 20 wherein said glass exhibits a linear coefficient of thermalexpansion between 32-40×10⁻⁷ /° C.
 25. In a flat panel display deviceaccording to claim 24 wherein silicon chips are mounted directly ontosaid glass utilizing chip-on-glass technology.
 26. In a flat paneldisplay device according to claim 24 wherein said glass is essentiallyfree from alkali metal oxides and consists essentially, expressed interms of mole percent on the oxide basis, of

    ______________________________________                                        SiO.sub.2                                                                            65-69    Y.sub.2 O.sub.3     0-3                                       Al.sub.2 O.sub.3                                                                     10-12    MgO                 1-5                                       B.sub.2 O.sub.3                                                                       7-10    CaO                 3-9                                       TiO.sub.2                                                                            0-3      SrO                 1-3                                       Ta.sub.2 O.sub.5                                                                     0-3      BaO                 2-5                                       Nb.sub.2 O.sub.5                                                                     0-3      MgO + CaO + SrO + BaO                                                                             11-16                                     ______________________________________                                         wherein the ratio Al.sub.2 O.sub.3 :B.sub.2 O.sub.3.>1.


27. In a flat panel display device according to claim 26 wherein theliquidus temperature of said glass in no higher than 1125° C.
 28. In aflat panel display device according to claim 26 wherein the viscosity ofsaid glass at the liquidus temperature thereof is greater than 60,000Pa.s.
 29. In a flat panel display device according to claim 26 whereinsaid glass exhibits a melting viscosity of about 20 Pa.s at atemperature below 1675° C.
 30. In a flat panel display device accordingto claim 26 wherein silicon chips are mounted directly onto said glassutilizing chip-on-glass technology.